Here you will find books by two or more collaborating authors (unless they use a combined pen name, in which case they'll be under that in Authors).
This is only the case in single contiguous stories. If you are looking for anthologies by various authors, look in Anthologies. Dumbass.
Books are listed here ALPHABETICALLY BY TITLE and I've included some pointers for series and franchises.
A (Wheel of Time, Aliens, Predator, Marvel Comics)
B (DC Comics, Belgariad)
C (Marvel Comics)
D (Riftwar, DC Comics, Marvel Comics, Doctor Who, Dragonlance)
G (DC Comics)
H (Warhammer, Marvel Comics, Riftwar, Dune)
J (Riftwar, DC Comics, 2000AD, Aliens)
L (DC Comics)
M (Riftwar, Marvel Comics, DC Comics)
O (Marvel Comics)
S (Riftwar, Star Trek, Star Wars, DC Comics)
T (DC Comics, 2000AD, Wheel of Time, Fighting Fantasy, Marvel Comics, Transformers)
U (Marvel Comics)
W (Marvel Comics, DC Comics)
X (Marvel Comics)
Y (Indiana Jones, DC Comics)